Okay so when he puts on his dress uniform to go make his first visit to the Sioux he has a red sash on. What's up with that? Wasn't he Infantry in the CW? And now I'm guessing he's a Cavfag. So where's the red come in?!?!?
According to this: http://www.ushist.com/wardrobe/q-1901u_union-army_officers_sash_civil-war_us.htm it's a crimson or wime color and was worn by line officers (Confederate Cav pukes wore yellow). I figure it was one of those things that the book authorized but was probably ditched early on by officers that would really have been fighting and kept by REMFs and Staff Weenies.
He was cavalry the whole time. JCD gave the reason for the sash above. Branch colors were worn as stripes on the uniform pant legs, hence the terms "redlegs" (artillery), "yellowlegs" (cavalry) and "spreadlegs" (infantry).
There was a saying during the Civil war, "who ever saw a dead cavalryman?" Just saying that Cavalry and their Armor decendants have yellow as their branch color for a reason.